1. Technical Field of the Invention
The present invention relates to drive circuits for light emitting diodes, and more particularly to a drive circuit for an array of light emitting diodes operable to maintain substantially constant brightness regardless of the number of light emitting diodes within the array which have been turned on.
2. Description of Related Art
Reference is now made to FIG. 1 wherein there is shown a light emitting diode (LED) array 10 and drive circuit 12 in accordance with the prior art. The LED array 10 is comprised of an N×M array of individual light emitting diodes 14. The reference M refers to a number of rows in the array 10, and more generally refers to a number of grids G of LEDs 14 which are included in the array. The reference N refers to a number of columns in the array 10, and more generally refers to a number of segments S (or individual LEDs 14) within each row or grid G of the array. As an example, the array 10 may include thirteen segments S (N=13) (or LEDs 14) in each of seven included grids G (M=7). The specific configuration with respect to only the first grid G (M=1) of the array 10 and its N LEDs 14 is shown in order to simplify the illustration. Each LED 14 includes a series connected current limiting resistor 16 in accordance with standard LED circuit design.
The LEDs 14 of the array 10 are connected in a common cathode configuration. Thus, within each grid G, the N included LEDs 14 all have their cathode terminals connected together. The common cathode connection node 18 for the LEDs 14 in each grid G is connected to a low side driver 20 comprised of, for example, an MOS transistor 22 (shown here as an n-channel device) having its source/drain terminals connected between a ground reference voltage 24 and the node 18. Thus, one low side driver 20 is provided for each grid G. A gate terminal of the transistor 22 is connected to receive a grid control signal output from a grid output latch circuit 26. This grid control signal in effect selects, through the corresponding low side driver 20, which one of the M grids G is to be actuated at a given time (and thus allow for segment S LED 14 illumination within that selected grid).
All of the LEDs 14, through their associated current limiting resistors 16, are connected to a high side driver 30 comprised of, for example, N in number MOS transistors 32 (shown here as n-channel devices). Each included high side driver 30 transistor 32 has its source/drain terminals connected between a positive reference voltage 34 and the current limiting resistors 16 associated with one LED 14 in each of the M grids G. Thus, a certain transistor 32 of the high side driver 30 is shared among and between M LEDs 14 in the included grids. For example, a first transistor 32(1) has its drain terminal connected to each of the resistors 16(1) for the LEDs 14(1) in each of the M grids G. Similarly, a second transistor 32(2) has its drain terminal connected to the resistors 16(2) for the LEDs 14(2) in each of the M grids G. This connection architecture is repeated across the N included LED 14 segments S of the M grids G within the array 10 and is schematically represented through the illustrated high side driver bus 46. A gate terminal of each transistor 32 is connected to receive a segment control signal output from a segment output latch circuit 36. These segment control signals in effect select which ones of the N LED 14 segments S (within the grid control signal selected grid G) is to be actuated. The segment control signals output from the segment output latch circuit 36 may be amplified and/or buffered and/or inverted by circuit 38 if desired/needed prior to application to the gate terminals of the transistors 32 of the high side driver 30.
It is important that the driver 12 for the array 10 be capable of maintaining a constant brightness across the array of LEDs. To achieve this goal, the voltage applied across an LED 14 and its associated series connected current limiting resistor 16 must be constant regardless of the number of other LEDs that have also been turned on. In the typical common cathode array architecture shown in FIG. 1, each high side driver 30 transistor 32 drives one LED 14 (within the selected grid G), and the low side driver 20 for that selected grid must sink the sum of the currents for all of the LEDs 14 within the grid which have been actuated. With N LEDs 14 per grid G, the low side driver 20 with the common cathode connection at node 18 may have to sink current for any of 1 to N LEDs 14. If the low side driver 20 transistor 22 is a MOS transistor, the voltage drop across this low side output would equal the sunk current from the actuated LEDs 14 times the on resistance of MOS device. In the FIG. 1 configuration for the array 10 and driver 12, a significant difference in voltage drop can occur, where this drop is dependent on the number of actuated LEDs 14 in the selected grid G. For example, assume that the on resistance of the transistor 22 is 1 Ohm, and the current per actuated LED 14 is 50 mA. With only one LED 14 actuated in the selected grid G, the voltage drop across the low side driver 20 would be 50 mV. However, with N=13 LEDs 14 actuated in the selected grid G, the voltage drop across the low side driver 20 would be 650 mV. This 600 mV difference between having one LED actuated and having thirteen LEDs actuated in the selected grid G could cause a noticeable difference in brightness between grids G having different numbers of actuated LEDs 14.
A need accordingly exists for an LED arrays driver to address the foregoing problem and maintain substantially constant brightness among and between LEDs across the grids of the array.